1. Field of the Invention
The present invention relates to a color encoder that is used in a digital image reproduction apparatus such as a video-CD or CD-G player so as to serve as an interface between such an apparatus and a television receiver.
2. Description of the Prior Art
A color encoder is used to obtain a color television signal from digital RGB signals reproduced from a disk. To achieve this, the color encoder converts the RGB signals into R-Y and B-Y color-difference signals, modulates a chrominance subcarrier with these color-difference signals to produce a modulated signal, and then inserts the chrominance subcarrier into the modulated signal at its color burst positions. The chrominance subcarrier is generated within the color encoder.
In a conventional color encoder, the chrominance subcarrier generator is provided with an address decoder and a data table. At every leading edge of the clock supplied from the outside, the chrominance subcarrier generator outputs, from the data table, the data stored at the address specified by the address decoder at that moment. The data outputted at predetermined time intervals in this way is then formed into a signal having a continuous waveform, and thus the chrominance subcarrier is produced. To achieve this, the data table is arranged in consideration of the frequency (hereinafter referred to as the clock rate) of the clock supplied from the outside and the desired frequency (hereinafter represented as f.sub.sc) of the chrominance subcarrier. In other words, the data table is so arranged that a chrominance subcarrier having a desired frequency f.sub.sc can be obtained by supplying from the outside a clock having a predetermined clock rate.
In recent years, various digital image reproduction apparatus such as video-CD and CD-G players have emerged, and apparatus of different reproduction systems use different clock rates. For example, video-CD players use a clock rate of 13.3 MHz, and CD-G players use a clock rate of 14.318 MHz. As for video-CD players, models using a clock rate of 27.0 MHz are now on their way to superseding models using the 13.5 MHz clock rate. In this way, even digital image reproduction apparatus of the same reproduction system have come to use more than one clock rate.
On the other hand, different television systems are used in different areas of the world, and each television system requires a different f.sub.sc frequency. For example, the NTSC system adopted in countries like the United States and Japan uses an f.sub.sc frequency of 3.58 MHz, whereas the PAL system adopted in many European and Southeast Asian countries uses an f.sub.sc frequency of 4.43 MHz.
However, since conventional color encoders are provided with only one data table, they can simply produce a chrominance subcarrier having one specific frequency from a clock having a specific frequency. That is, conventional color encoders can only cope with a combination of one specific clock rate and one specific f.sub.sc frequency. Accordingly, to use digital image reproduction apparatus of different reproduction systems in combination with television receivers of different television systems, it is necessary to use as many color encoders as the number of possible combinations of the reproduction systems and the television systems.